Behavior of human apolipoprotein E in aqueous solutions and at interfaces.

Human plasma apolipoprotein E (apo-E) strongly self-associates to form a stable tetramer in an aqueous solution at pH 7.4 containing 0.15 M NaCl. Tetramerized apo-E is abundant in alpha-helical conformation with an asymmetric molecular shape. Apo-E forms a stable monolayer at the air-water interface with a collapse pressure of 14 dynes/cm and with a limiting molecular area of 21 A2/amino acid. Under low surface pressure (less than 0.5 dyne/cm), it behaves as a monomer at the interface. It binds reversibly to the surface of phosphatidylcholine-coated triolein particles with a diameter of 26 nm from the aqueous phase in which most of the molecules are tetramerized. An apparent dissociation constant (Kd), 1.2 X 10(-6) M (monomeric molarity) or 40 mg/l, is substantially larger than those of the other apolipoproteins, while a binding saturation level (N), 0.8 amino acid/surface phospholipid, is equivalent to the N values of those proteins (Tajima, S., Yokoyama, S., and Yamamoto, A. (1983) J. Biol. Chem. 258, 10073-10082). Content of alpha-helix increases slightly when it is transferred from the aqueous phase to the lipid surface. The results are consistent with a model that amphiphilic alpha-helical conformation is responsible both for self-association and surface binding and suggest that apo-E may easily dissociate from the lipoprotein surface to form a self-associated soluble tetramer.